1. Field of the Invention
The present application relates to the use of modifiers for a creping adhesive used in the production of creped paper.
2. Description of the Related Art
Softness of a paper product, such as a tissue or towel, is a desirable attribute. Softness, like strength and absorbency, plays a key role in consumer preference. Softness relates both to the product bulk and surface characteristics. Softness is the tactile sensation perceived by a user when they touch and hold the paper product.
Paper is generally manufactured by suspending cellulosic fibers of appropriate length in an aqueous medium and then removing most of the water from the resulting web. The paper derives some of its structural integrity from the mechanical arrangement of the cellulosic fibers in the web, but most of the paper's strength is derived from hydrogen bonding, which links the cellulosic fibers to one another. The strength imparted by this interfiber bonding, while necessary to the utility of the product, results in a lack of perceived softness that is inimical to consumer acceptance.
One method of increasing the softness of paper is by creping it. Creping, by breaking a significant number of interfiber bonds, increases the perceived softness of the resulting product. Creping processes are well known in the art. The fibrous structure of the paper is mechanically foreshortened in the machine direction in order to enhance bulk, stretch, and softness. The fibrous web is adhered to a dryer, for example, a Yankee dryer, and removed from the dryer using a flexible creping blade. The terms “creping blade,” “crepe blade,” and “doctor blade” are used interchangeably herein. The creping blade can be made of metal, ceramic, or other materials known in the art. The degree to which the web is adhered to the dryer is a factor in determining how uniform the creping will be, and thus, the bulk, stretch, and softness of the creped web.
Creping aids are applied to a creping dryer surface to facilitate the creping process. Creping aids can comprise creping adhesives, creping modifiers, other creping additives, and/or combinations thereof. The adhesion level of the web to the dryer surface is important, since it relates to the controllability of the web from the creping blade to the reel on which the paper is wound. Paper webs not sufficiently adhered to a creping dryer surface are difficult to control and can cause wrinkles and weaving of the web in the parent roll. When a web weaves at the reel, the parent roll edges are uneven. Poorly creped webs not only affect the reliability of the papermaking operation, but also can cause sheet breaks and difficulties in converting base sheet into finished product rolls of towel or tissue.
The level of adhesion of a web to a creping dryer surface is also important because it relates to the transfer of heat from the surface of the dryer to the web and ultimately affects the drying rate. Therefore, higher levels of adhesion allow for a web to dry faster, thus allowing the paper machine to operate at higher speeds.
A through-air-dried web tends to have poorer adhesion to a creping dryer surface than a conventionally wet pressed web. There are several reasons for this phenomenon. First, through-air-dried webs contact the surface of a creping dryer at lower contact levels since the web is transferred to the surface of the creping dryer with a limited-knuckle-area fabric, while a conventionally wet-pressed web is pressed more uniformly with a felt against the dryer surface. Second, through-air-dried webs are transferred to a creping dryer surface at higher dryness levels, while conventionally wet-pressed webs are transferred at lower dryness levels. The lower dryness level facilitates more intimate contact of the web with the dryer surface and, hence, better adhesion.
It is important that the creping aids have the proper softness/flexibility to allow sheet adhesion yet allow a doctor blade to maintain a clean creping dryer surface. For example, if a creping adhesive becomes too hard, incomplete removal of adhesive from the creping surface can occur and portions of the web may remain adhered to the creping dryer surface. When portions of the web remain adhered to the creping dryer, defects often result in the web, ultimately leading to poor quality products and breaks in the web in the open draw between the creping doctor and reel.
Excessive build-up of creping adhesive on the creping dryer surface is another problem associated with the use of creping aids, for example, producing streaky dryers. The streaks on the dryer impact the profile of adhesion in the cross-direction (CD), or width direction, of a paper machine, often resulting in reels with bumps or wrinkles. The usual remedy is to change creping blades; however, changing the blades leads to downtime of the paper machine, and creping blades are costly. Alternatively, coating streaks can be controlled through the use of a cleaning blade, which is positioned after the creping blade on a creping dryer. The cleaning blade is frequently changed to control streaks and excessive adhesive build-up.